Heterogeneous reworking of Grampian metamorphic complexes during Scandian thrusting in the Scottish Caledonides: insights from the structural setting and U–Pb geochronology of the Strath Halladale Granite

The Neoproterozoic Moine Supergroup in the Caledonides of northern Scotland is disposed in a series of thrust nappes. The Skinsdale Thrust in East Sutherland separates Moine rocks of the Loch Coire Migmatite Complex, which underwent anatexis during the Ordovician Grampian phase of the Caledonian orogeny, from the generally unmigmatized Scaraben succession. The Strath Halladale Granite occurs as an east-dipping sheeted complex that cuts discordantly at a low angle across the thrust. The pluton carries a magmatic foliation that was reworked by solid-state deformation at high to moderate temperatures; shear zones within the pluton display top-to-the-NW sense of shear similar to that deduced for the Skinsdale Thrust. Granite sheets are deformed by curvilinear folds that developed during ductile thrusting. Field and petrographic evidence is consistent with a thrust-related mode of emplacement. U–Pb monazite geochronology yields a crystallization age for the granite of 426 ± 2 Ma. This is interpreted to broadly date displacement along the Skinsdale Thrust, which therefore is of similar age to other NW-directed Silurian ductile thrusts within this sector of the Caledonides. These findings substantiate a model involving Ordovician (Grampian) deformation and metamorphism followed by Silurian (Scandian) thrust-related reworking and emplacement of nappes onto the Laurentian foreland.